The present invention relates to a method for restoring and/or enhancing the material properties of a dielectric material. More particularly, the invention relates to a method for restoring hydrophobicity to the surfaces of low dielectric films which have been subjected to an etching or ashing treatment in such a way as to remove at least a portion of previously existing carbon containing moieties, resulting in a film having reduced hydrophobicity and increased dielectric constant. Such films are used as insulating materials in the manufacture of semiconductor devices such as integrated circuits (“ICs”).
As semiconductor devices scale to lower technology nodes, the requirement for lower and lower dielectric constant (k) has been identified to mitigate RC delay. Similarly, as feature sizes in integrated circuits are reduced, problems with power consumption and signal cross-talk have become increasingly difficult to resolve. To achieve lower k (2.6 to 3.0) in dense inorganic materials, carbon has been added to reduce the polarizability thus reducing k. To achieve ultra low k (<2.4) materials, porosity is typically added to the carbon-rich dense matrix. While the introduction of carbon and porosity have reduced k, new challenges during back end of the line (BEOL) processing have also been identified. Specifically during processes such as, for example, etching and ashing, reactive gases have been found to damage the carbon at the surface of dense materials. Other processes such as, for example, wet chemical stripping, CMP, and post-CMP cleaning are also known to damage the surface carbon. Porous low k's suffer more catastrophic effects from reactive etch and ash gases due to diffusion through the film, which causes a greater extent of damage throughout the film. Once the carbon has been removed from the films, the films react with atmospheric moisture and hydroxylate. These hydroxyls adsorb and hydrogen bond with water. Because water has a dielectric constant of about 70, small amounts that are absorbed for dense materials and adsorbed for porous materials cause the dielectric constant to increase significantly. Also, porous materials tend to void after copper annealing due to the high tensile stress fields which will destroy device yields.
It is believed that the integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications will help to solve these problems. While there have been previous efforts to apply low dielectric constant materials to integrated circuits, there remains a longstanding need in the art for further improvements in processing methods and in the optimization of both the dielectric and mechanical properties of such materials. Device scaling in future integrated circuits clearly requires the use of low dielectric constant materials as a part of the interconnect structure. Most candidates for low dielectric constant materials for use in sub-100 nm generation ICs are carbon containing SiO2 films formed by either CVD or spin-on methods. During subsequent processing steps, such as plasma etching and photoresist removal using plasma or wet strip methods, significant damage occurs to these low-k materials, which causes fluorine addition and carbon depletion from the low-k material adjacent to the etched surface. In addition to a higher effective k, the resultant structures are susceptible to void formation, outgassing and blister formation. The voids in turn may cause an increase in leakage current at elevated voltages and reduction in breakdown voltage. Accordingly, there is a desire in the art to repair damage caused to a porous SiCOH-based low-k material.
One way to approach this challenge is to repair the damaged area on dense surfaces, or in the case of porous materials on the surface of the film as well as the internal pore walls with a re-methylating compound called a restoration agent. Restoration agents react with the damaged hydroxylated surfaces to re-alkylate or re-arylate them which in-turn restores the dielectric constant. The following reaction describes an exemplary re-methylation process: SiOH (damaged surface)+RxSi(Cl)y (restoration agent) yields SiOSiRx (repaired surface)+(HCl)y (hydrochloric acid). In the case of porous damaged internal pore wall surfaces, the re-methylation prevents void formation. Many times, the use of a restoration agent allows for conventional etch, ash, and wet cleaning processes to be utilized with low and ultra low dielectric constant materials. The treatment could result in replenishment of carbon to the low-k film, thereby restoring hydrophobicity and resistance to further damage. Additionally, it would be desirable if the repaired low-k material was found to be resistant to void formation, which generally occurs in untreated porous low-k inter-level dielectric regions during copper annealing processes. Re-methylating compounds or silylating agents (which are examples of restoration agents) can methylate the surface of SiO2 based materials. Contemplated exposure includes vapor exposure (with or without plasma), aerosol exposure, spin coating and supercritical CO2. Normally, organosilicate glass (OSG) porous low-k materials are susceptible to void formation in ILD during Cu damascene processing. After treatment with a restoration agent, the resulting structure is significantly more resistant to void formation. Without being bound to any specific theory or mechanism, it is believed that plasma damage causes carbon depletion in the dielectric, by replacing Si—CH3 bonds with Si—OH or SiH bonds depending upon the type of plasmas used, e.g oxidizing and reducing, respectively. In damaged porous dielectrics, the pore surface is now covered with Si—OH bonds. In the presence of tensile stress (such as after Cu annealing), adjacent Si—OH groups can condense, thus causing local densification. The evolving reaction products and the stretching of the molecules due to the new links formed, causes voids to occur near the center of the ILD space. Restoration agents prevent void formation by replacing most Si—OH bonds by Si—O—Si—Rx bonds, which avoid condensation reactions. Therefore void formation does not occur.
Treatment with a restoration agent performed after dielectric trench and via formation using etching, ashing, and wet chemical processes repairs carbon depletion and damage to the low-k materials. By this means, voids are deterred and the low-k materials can withstand internal stresses caused by annealing treatments to the metal filling the trenches and vias.
Treatment with a restoration agent is typically conducted by exposing the wafer surface to the silylating agent in liquid or gas form for a period sufficient to complete the reaction with the damaged low-k region. Optionally, a high temperature bake can be performed to remove remaining solvent and excess restoration agent. Also, optionally, a wet cleaning operation can be performed immediately before the restoration agent application using a material that is chemically compatible with the low-k dielectric material. Additionally a dehydration bake may be performed before the restoration agent treatment to increase effectiveness of the restoration agent.
The effectiveness of the restoration agent can be verified using unpatterned low-k dielectric films subjected to etching and ashing processing followed by treatment with the restoration agent. A successful treatment with a restoration agent results in increased carbon concentration that can be measured by FTIR, EDX, SIMS, or XPS techniques. Additionally, a water contact angle increase is seen after the application of the restoration agent, which demonstrates the hydrophobic nature of the post-treated surface. The restoration agent treated film also shows a lower dielectric constant extracted from C—V measurements, compared to an etched/ashed film that is not treated with restoration agent. In patterned wafers, the effectiveness of the restoration agent treatment is demonstrated by reduction or elimination of voids in the low-k dielectric in narrow spaces between Cu trenches after a copper anneal treatment following electroplating of copper, and also by lower profile change in trenches or vias after exposure to reactive solvents.
U.S. patent application Publication No. 2006/0057855 A1 to Ramos et al. (“the 855 publication”) discloses a “toughening agent” composition for increasing the hydrophobicity of an organosilicate glass dielectric film when applied to said film. According to the 855 publication, the toughening agent includes a component capable of alkylating or arylating silanol moieties of the organosilicate glass dielectric film via silylation, and an activating agent selected from the group consisting of an amine, an onium compound and an alkali metal hydroxide. The 855 publication discloses that the toughening treatment is conducted by exposing the wafer surface to the silylating agent in liquid or gas form for a period sufficient to complete the reaction with the damaged low-k region. The 855 publication further discloses that the toughening treatment can also be conducted in the presence of a plasma derived from, for example, a silane compound, however, no such procedure is exemplified. The use of such plasma in a restoration process, however, is likely to suffer from significant drawbacks.
Plasma chemistry is a useful methodology employed in the manufacture of integrated circuit and other electronic devices to deposit and modify film chemistry for a variety of functions within the layers of dielectric material. Plasmas are employed, for example, to deposit interlayer dielectric materials, barrier materials, and capping materials. Other uses include the modification of surfaces using oxidative or reductive atmospheres to increase the surface roughness or change the chemistry in the surface to increase the adhesion between two films, e.g., metal barriers to interlayer dielectric materials or capping materials to copper lines. For the restoration of low dielectric materials after RIE, ashing, and wet cleaning, plasmas may not be the best solution.
Many of the chemistries used to repair dielectric materials will deposit a film under thermal or plasma enhanced CVD processes. Deposition is not necessarily desirable since it may affect the critical dimensions of the feature and may cause issues during packaging due to adhesion and cracking. Similarly, the pore size of many of the dielectric materials ranges from 10-30 Å, therefore small molecules of this dimension are required to insure that the restoration penetrates the damaged portions of the film. Since plasmas are energetic energy sources, there may be gas phase polymerization of the restoration chemistry resulting in molecular sizes greater than the pore size of the dielectric material. These polymerized species will only react at the upper surfaces of the film and not restore the electrical and composition of the entire damaged layer. Two other potential issues with the use of plasmas are: the plasma may cause additional damage to the film due to ion bombardment; and the plasma relies on the ions and other neutral species in the line of site of the features being formed and may minimally interact on the sidewalls of the trenches and vias where it is crucial to repair the damage, i.e., diffusion of plasma generated species may be slow. Plasmas also have the ability to roughen surfaces which may not be desirable for the sidewalls of trenches and vias.
Moreover, with either gas phase or liquid phase restoration processes, a potential exists for leaving residue or chemical species trapped within the dielectric material, particularly during BEOL processing. Although the surfaces of the film are made hydrophobic and the dielectric constant restored by applying silylating chemistries, the silylating species is known to become trapped in the dielectric layer followed by unwanted outgassing during subsequent processing steps, especially those steps that occur at elevated temperatures such as, for example, thermal cycling between temperatures of about 50° C. and about 450° C. Such outgassing causes defects (e.g., pinholes, adhesion, and delamination) created by vapors escaping from the dielectric material during the deposition of metal barriers and capping layers which typically leads to the re-adsorption of water, copper migration into the dielectric layer, and adsorption of other atmospheric contaminants. Each of these issues will cause decreased reliability and modify the performance of the final device. Accordingly, there is a need in the art for a method for restoring dielectric properties of a dielectric material that does not suffer from the above-identified drawbacks.